: A large number of genes coding putative olfactory receptor proteins have been identified, but without a suitable system for expression, the odors which activate each of these receptors remain unknown. As a means of relating odorants to specific receptors, a series of experiments are proposed to identify odors that maximally activate receptor cells in the various expression zones in the olfactory epithelium identified by in situ hybridization studies and then to define the molecular characteristics of the odors that determine the responses. EOG recordings from localized regions of the olfactory epithelium will be used instead of recording from single receptor cells because of the low probability of finding cells with identical response properties. The issue of receptor diversity within each expression zone will be approached by using cross-adaptation to selectively depress responses mediated by some receptors. Localized stimulation of the epithelium will be used to determine if there are regions of epithelium that excite or inhibit particular types of neurons in the olfactory bulb. The responses of bulbar neurons will be assessed by intracellular recording and the cells morphologically identified by dye marking. These experiments will offer insights into the relationship between chemical structure and smell, a sensory modality important in appetite, social behavior and endocrine function.